The subject matter disclosed herein relates to a system for providing zone selective interlock between circuit breakers in an electrical distribution system. In particular, the subject matter disclosed herein relates to a system for zone selective interlock between a downstream circuit breaker and an upstream circuit breaker during instantaneous trip conditions.
Air circuit breakers are commonly used in electrical distribution systems. A typical air circuit breaker (“ACB”) comprises an assembly of components for connecting an electrical power source to a consumer of electrical power called a load. The electric circuit the circuit breaker is connected to is referred to herein as the protected electric circuit. The components are referred to as a main contact assembly. In this assembly, a main contact is typically either opened, interrupting a path for power to travel from the source to the load, or closed, providing a path for power to travel from the source to the load. The ACB will further include devices and sensors, referred to as a trip unit, capable of sensing the current flowing through to the protected circuit and comparing the magnitude of the sensed current against predetermined thresholds. The trip unit's thresholds and current magnitude trip characteristics is displayed in a graphical form, commonly referred to as a “trip curve,” which indicates to a user how long the trip unit will take to trip under any given overload condition. Upon detection of an overcurrent fault condition on the protected circuit outside of the trip curve parameters, the trip unit activates the mechanical linkages that cause the main contact assembly to open.
Downstream from the ACB, other ACBs or, one, or more smaller capacity circuit breakers, sometimes referred to as molded case circuit breakers (“MCCB's”), may be installed to further protect and isolate portions of the electrical distribution system. The MCCB's are available in multiple capacities and are often arranged in tiers or levels, with multiple MCCB's being arranged within each tier. The MCCB's, similar to the air circuit breaker, include a main contact assembly that is opened and closed via a mechanical linkage assembly. A trip unit associated with each MCCB senses current flowing through that portion of the protected circuit. Similar to the ACB, the MCCB utilizes trip curves to determine when a given condition is outside the desired parameters.
Since both the ACB and the MCCB react to fault conditions, circumstances may arise where an electrical fault causes both the ACB and MCCB to open their respective contact assemblies. In general, it is desirable to have the circuit breaker closest to the fault interrupt the fault current, commonly referred to as tripping, such that the smallest portion of the protected circuit, closest to the fault is interrupted. The portion of the circuit bounded by its source circuit breakers and any circuit breakers exiting the circuit is called the zone of protection for the source circuit breakers. If the zone becomes faulted it is called the faulted zone. Interrupting the fault current close to the fault prevents, or at least minimizes the amount of disruption on other operations connected to other protected circuits. To avoid having the upstream ACB or an upstream MCCB trip, the system of circuit breakers on the protected circuit is coordinated. Through selective coordination, the trip curves of each circuit breaker are adjusted to allow the downstream circuit breaker the opportunity to trip first without jeopardizing the functionality of the circuit breaker system above the faulted zone.
Circuit breaker systems may also incorporate a zone selective interlock (“ZSI”) arrangement. In a ZSI system, the downstream circuit breaker communicates with an upstream circuit breaker when a fault is detected. The upstream circuit breaker then changes its tripping response to allow the downstream circuit breaker to clear the fault. Thus, the upstream circuit breakers have two modes of operation. These modes are commonly referred to as “restrained” and “unrestrained” operation. In an unrestrained mode, the circuit breaker operates under the fastest default trip curve and is not modifiable by the operator. Due to timing issues in the communications between the circuit breakers, the fastest trip curve is used to ensure adequate protection. When the communication of a fault is received by the upstream circuit breaker, the upstream circuit breaker switches to the restrained mode of operation. In the restrained mode, the upstream circuit breaker will shift the trip curve to a curve set by the operator, this has the effect of delaying the tripping response of the upstream circuit breaker. The switching of the trip curve allows the upstream circuit breaker to perform its function while also allowing the downstream circuit breaker to clear the fault. The zone selective interlock system thus allows the operator to minimize the impact of the fault without jeopardizing the functionality of the circuit breaker system.
While existing circuit breakers coordinated using cascaded time delays are suitable for their intended purposes, there still remains a need for improvements particularly regarding the ability of the operator to define the trip curves and the coordination of the circuit breakers when operating in ZSI restrained modes.